What I recall reading a few years ago is that NASA thought it was a good way to reduce noise.
Some say those silent black helicopters circling overhead use this technology.
Old Thread, see post #8
Fan & Propeller shapes: Sickle and Curved
Quote:
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Trailing Edge Blowing blade with top skin removed. Air enters at the retainer (bottom right) and exits through turning vanes at the trailing edge (left).
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